Abstract
Physiology is the study of the functions of the body, its organs and the cells of which they are composed. It is often said that physiology concerns itself with maintaining the status quo or ‘homeostasis’ of bodily processes. However, even normal physiology is not constant, changing with development (childhood, pregnancy and ageing) and environmental stresses (altitude, diving and exercise). Physiology might be better described as maintaining an ‘optimal’ internal environment; many diseases are associated with the disturbance of this optimal environment.
Physiology is the study of the functions of the body, its organs and the cells of which they are composed. It is often said that physiology concerns itself with maintaining the status quo or ‘homeostasis’ of bodily processes. However, even normal physiology is not constant, changing with development (childhood, pregnancy and ageing) and environmental stresses (altitude, diving and exercise). Physiology might be better described as maintaining an ‘optimal’ internal environment; many diseases are associated with the disturbance of this optimal environment.
Anaesthetists are required to adeptly manipulate this complex physiology to facilitate surgical and critical care management. Therefore, before getting started on the areas of physiology that are perhaps of greater interest, it is worth revising some of the basics – this chapter and the following four chapters have been whittled down to the absolute essentials.
How do the body’s organs develop?
The body is composed of some 100 trillion cells. All life begins from a single totipotent embryonic cell, which is capable of differentiating into any cell type. This embryonic cell divides many times and, by the end of the second week, gives rise to the three germ cell layers:
Ectoderm, from which the nervous system and epidermis develop.
Mesoderm, which gives rise to connective tissue, blood cells, bone and marrow, cartilage, fat and muscle.
Endoderm, which gives rise to the liver, pancreas and bladder, as well as the epithelial lining of the lungs and gastrointestinal (GI) tract.
Each organ is composed of many different tissues, all working together to perform a particular function. For example, the heart is composed of cardiac muscle, conducting tissue, including Purkinje fibres, and blood vessels, all working together to propel blood through the vasculature.
How do organs differ from body systems?
The organs of the body are functionally organised into 11 physiological ‘systems’:
Respiratory system, comprising the lungs and airways.
Cardiovascular system, comprising the heart and the blood vessels. The blood vessels are subclassified into arteries, arterioles, capillaries, venules and veins. The circulatory system is partitioned into systemic and pulmonary circuits.
Nervous system, which comprises both neurons (cells that electrically signal) and glial cells (supporting cells). It can be further subclassified in several ways:
– Anatomically, the nervous system is divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), consisting of peripheral nerves, ganglia and sensory receptors, which connect the limbs and organs to the brain.
– The PNS is functionally classified into an afferent limb, conveying sensory impulses to the brain, and an efferent limb, conveying motor impulses from the brain.
– The somatic nervous system refers to the components of the nervous system under conscious control.
– The autonomic nervous system (ANS) regulates the functions of the viscera. It is divided into sympathetic and parasympathetic nervous systems.
– The enteric nervous system is a semiautonomous system of nerves that control the digestive system.
Muscular system, comprising the three different types of muscle: skeletal, cardiac and smooth muscle.
Skeletal system, the framework of the body, comprising bone, ligaments and cartilage.
Integumentary system, which is essentially the skin and its appendages: hairs, nails, sebaceous glands and sweat glands. Skin is an important barrier preventing invasion by microorganisms and loss of water (H2O) from the body. It is also involved in thermoregulation and sensation.
Digestive system, including the whole of the GI tract from mouth to anus and a number of accessory organs: salivary glands, liver, pancreas and gallbladder.
Urinary system, which comprises the organs involved in the production and excretion of urine: kidneys, ureters, bladder and urethra.
Reproductive system, by which new life is produced and nurtured. Many different organs are involved, including the ovaries, testes, uterus and mammary glands.
Endocrine system, whose function is to produce hormones. Hormones are chemical signalling molecules carried in the blood that regulate the function of other, often distant cells.
Immune system, which is involved in tissue repair and the protection of the body from microorganism invasion and cancer. The immune system is composed of the lymphoid organs (bone marrow, spleen, lymph nodes and thymus), as well as discrete collections of lymphoid tissue within other organs (for example, Peyer’s patches are collections of lymphoid tissue within the small intestine). The immune system is commonly subclassified into:
– The innate immune system, which produces a rapid but non-specific response to microorganism invasion.
– The adaptive immune system, which produces a slower but highly specific response to microorganism invasion.
The body systems do not act in isolation; for example, arterial blood pressure is the end result of interactions between the cardiovascular, urinary, nervous and endocrine systems.
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